What is an explicit-null label

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The explicit-null label is label 0, which takes effect only at the bottom of the stack. An LSR must pop up the label stack and forward the explicit-null label based on the IPv4 header information.

The penultimate hop attaches label 0 to the bottom of the stack and forwards the stack to the egress node. The egress node does not search in the ILM if a packet has label 0. It pops the label 0 and determines the forwarding path based on the IP header information (if the label stack has only one layer).

Other related questions:
What is an implicit-null label
The implicit-null label is label 3, which is used and assigned by an LSR. The implicit-null label, however, is not used in encapsulation. To switch the label, an LSR needs to pop up the label stack upon finding that the new label is an implicit-null label. If the downstream forwarding entry uses label 3, the penultimate hop directly sends packets to the egress node without attaching any label. The egress node follows the IP forwarding process to forward the packets as if the packets contain no MPLS label.

How to configure the egress node to assign explicit-null labels to the penultimate hop
If the label of the penultimate hop is implicit-null, the value of the label that the egress node assigns to the penultimate hop is 3. If the label of the penultimate hop is non-null, the PHP feature is not used and the egress node properly assigns labels to the penultimate hop. The value of the label is equal to or larger than 16. Typical failure: The label distribution mode is not modified, or the label distribution mode is set to non-null. Solution: In the MPLS view, set the label distribution mode to explicit-null.

In which conditions does LDP use non-null labels
LDP uses non-null labels in any of the following situations: Non-null labels are configured in the MPLS view. Upon receiving a Label Mapping message, the egress node advertises non-null labels if it does not respond with a Label Release message, regardless of whether the LSP is a liberal LSP. A transit LSP is established. On a remote LDP session, the LSP triggering mode is all if no downstream local session exists. The LSR functions as the agent egress node to assign non-null labels to FECs along the local route.

What types of LDP labels are there
LDP labels include: Explicit-null label Implicit-null label Non-null label

Displaying the e-label on the firewall
Perform as follows to view the e-label on the firewall: display elabel Syntax display elabel [ slot-id ] Parameter description slot-id specifies the ID of the slot where the board is inserted. Example Display the e-label of the board in slot 0. display elabel 0

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